Referring to FIG. 1, a circuit 10 represents a conventional RF power amplifier. The circuit 10 includes a field-effect transistor (FET) 14 coupled to the supply voltage V.sub.s through an RF choke 12. The FET 14 receives an RF drive at its gate 15, thus performing as an RF periodic current sink or switch. A capacitor 16 is disposed between a node 13 and a node 18 for DC blocking purposes. A bandpass filter 19, comprising an inductor 20 and a capacitor 22 is disposed between the node 18 and the load resistance 26. A matching network 24 is disposed between the node 18 and the output 26 of the power amplifier 10, to transform the impedance of the load 28 to a desired output impedance (Z.sub.o) at node 18.
Ideal class C RF power amplifiers, or switch mode amplifiers (class D, E, or F) operate at an optimum output power (P.sub.o) determined by the power supply voltage (V.sub.s) and the operating load line (R.sub.L) by the equation: EQU P.sub.o =V.sub.s.sup.2 /2R.sub.L ( 1)
Thus, for a fixed voltage supply (as in portable radio amplifiers) output power is determined by the load line R.sub.L.
RF power amplifiers of conventional design typically suffer from a significant efficiency reduction when the output power level is adjusted to values below the peak design output power by varying the input drive to the power amplifier. This is true for all classes of amplifiers: A, B, C, E, and F. Maintaining efficiency with RF output power cutback is an important requirement for radios that are designed to save battery power as a result of reduced power operation. One known option for varying output power while maintaining efficiency is to adjust the voltage supply to the amplifier stage. However, this option is inconvenient since a high efficiency DC/DC converter is required in the case of a battery powered RF amplifier.